1. Field of the Invention
The present invention relates generally to high conductivity carbon fiber, and more specifically to resin or rubber composite including gaseous phase growth carbon fiber high in conductivity, dispersibility and moldability, and stable in resistivity.
2. Description of the Prior Art
With the advance of electronic technology, there exists a strong demand for conductive composite material composed of carbon fiber and plastic or rubber, as static electricity or electromagnetic wave shielding material, which is light in weight, strong in strength, high in conductivity and excellent in moldability.
Conventionally, carbon black has been mixed with resin material in order to obtain conductive resin composite material. However, in conductive resin composite material mixed with carbon particles such as carbon black, since carbon black structure is easily destroyed by shearing force produced when carbon black is mixed with resin or when the resin is molded into a predetermined shape, there exists a problem in that the electric resistivity easily changes and therefore it is impossible to keep a required conductivity.
In general, carbon fiber has been manufactured by heat-treating organic raw material fiber such as polyacrylonitrile or rayon at about 2500.degree. C. into carbon. Recently, however, it has been known that it is possible to obtain graphite fiber with ideal crystal structure and an improved conductivity by heat-treating gaseous phase growth carbon fiber at 2500.degree. C. or higher. The above gaseous phase growth carbon fiber can be obtained by vapor phase thermodecomposing hydrocarbon together with metal or metallic compound. Further, the gaseous phase growth carbon fiber has such a crystal structure that carbon hexagonal net planes are arranged substantially in parallel to the fiber axis and further in annular ring growth fashion around the fiber axis (e.g. in Chemical Engineering Transaction, Volume 50; No. 3, Pages 42 to 49, 1986). Further, the method of manufacturing gaseous phase growth carbon fiber without depending upon substrates has been disclosed (e.g. in Japanese Patent Kokai Publication Nos. 58-180615, 60-54998, and 61-218661).
In particular, in Japanese Patent Kokai Application No. 61-218661, carbon fiber with a fiber diameter of 0.05 to 4 .mu.m and an aspect ratio (length/dia. ratio in fiber) of 20 to 1000 is manufactured by introducing hydrocarbon and a specific organic metal compound or these together with a carrier gas into a reactive space for vapor phase thermodecomposition of the introduced hydrocarbon by catalyst reaction and further by heat-treating the obtained carbon fiber where necessary. The manufactured carbon fiber is mixed with rubber or plastic.
The above-mentioned carbon fiber is high in conductivity and little in crystal structure change as compared with the conventional carbon black or carbon fiber. In this improved carbon fiber, however, there still exist various problems in that carbon fiber entwines to each other when grown; the fiber length differs according to the reaction conditions; and therefore the dispersibility into resin or rubber is not uniform, so that it is difficult to put the fiber into practical use.
On the other hand, it has been proposed that it is possible to obtain intercalation compound with a very high conductivity by infiltrating reactive substances such as nitric acid or alkali metal compound to carbon net planes (in Japanese Patent Kokai Publication Nos. 59-179816 and 60-21806).
In the above-mentioned prior-art graphite intercalation compound, however, since carbon fiber manufactured by seeding fine metal particles on a substrate (i.e. on the basis of a substrate) is used as base material, there exists a problem in that it is difficult to form graphite intercalation compound.
In more detail, gaseous phase growth carbon fiber C can be shown in FIG. 1, in which both ends C.sub.1 of the fiber are semispherical and further the hollow portion is small in cross-sectional area. Therefore, when the carbon fiber is heat-treated, since the crystal structure of the carbon fiber C grows up without changing the entire crystal structure, there exists a problem in that reactive compound cannot easily inflitrate into the carbon fiber C and therefore it is difficult to form graphite intercalation compound.